1. Field of the Invention
The present invention relates generally to abrasion-resistant coatings and more particularly to such coatings that also are deformable. The coatings of the invention have sufficient deformability so that substrates coated therewith can be subsequently embossed.
2. Background of the Prior Art
Numerous compositions for forming abrasion-resistant coatings on various types of substrates are well-known in the art.
Silicone compositions of the above type are disclosed in U.S. Pat. No. 4,455,205 to Olson et al. (issued Jun. 19, 1984 and assigned to General Electric Company, Schenectady, N.Y.) The coatings comprise an ultraviolet light curable composition prepared from a multifunctional acrylate monomer, an acryloxyfunctional silane and aqueous colloidal silica in a polar solvent. After mixing the components, the solvent and remaining water is removed in vacuo, with gentle heating, a step which is termed "stripping." A photoinitiator is then added and the composition is cured.
Similarly, U.S. Pat. No. 4,486,504 to Chung, (issued Dec. 4, 1984 and also assigned to General Electric Company) discloses an ultraviolet light curable composition prepared from the addition of a multifunctional acrylate monomer to a mixture of acryloxyfunctional silanes, and/or glycidoxy functional silanes and aqueous colloidal silica. After stripping, a photoinitiator is added and the composition cured.
Such abrasion-resistant coatings are frequently applied over polycarbonate substrates. The coated substrates are often used as a substitute for glass, as in the case of lenses for automobile head lamps. In these applications, where there is little or no flexure or deformation of the coated substrate, the coating compositions of the prior art generally perform quite well, providing a scratch-resistant, transparent coating.
The prior art coating compositions have several drawbacks, however, relating to performance and ease of formulation.
When there is a need for repeated flexure or plastic deformation of the coated substrate, the prior art compositions tend to craze, crack and spall. This results in an unsightly surface finish and may leave the substrate unprotected.
A deformable, abrasion-resistant coating would be very useful in the production of electronic switch pads, such as those used in modern cash registers and the like. An embossing process is used on a polycarbonate sheet to delineate the various key positions. The embossed sheet is subjected to repeated flexure from manual depression that is employed to actuate electronic switches located below the key positions.
Because compositions for forming an abrasion-resistant coating are generally applied as a liquid that is subsequently cured, the polycarbonate sheet must be coated prior to embossing or a uniform coating of desired thickness cannot be achieved.
The abrasion-resistant coatings of the prior art are not deformable and generally behave in a brittle manner during embossing. Even those prior art coatings that survive embossing frequently wear at an excessive rate or crack or spall upon repeated flexure of the substrate.
Another drawback associated with the prior art abrasion-resistant coatings is the difficulty of formulation. For instance, in formulating the coating composition as taught in the above-mentioned U.S. Patent to Chung, the present inventors found that, upon stripping, the admixture of the components had a strong tendency to gel. This rendered a final product having a viscosity too high to produce an abrasion-resistant coating of optimum thickness or one wherein the colloidal silica agglomerated, yielding a chalky, nontransparent coating.
The present inventors have overcome the drawbacks of the prior art by providing a novel composition which forms a transparent, adherent, deformable and abrasion-resistant coating upon curing. Furthermore, the method of the present invention for formulating the composition avoids the gelling problems associated with the prior art compositions.